It has been for many years common practice in the art of geophysical exploration for oil, gas and other minerals to perform seismic exploration of the earth's subterranean structure. From knowledge of the shapes of various layers of differing types of rocks in the earth's crust, experienced geophysicists can deduce the possible presence of various valuable minerals such as oil and gas. Seismic techniques generally involve the generation of a source of acoustic energy such as a blast of dynamite, the dropping of a heavy weight on the earth's surface or imparting of a mechanical vibration to the earth, the so-called "Vibroseis" technique. The Vibroseis technique has grown increasingly popular in recent years with the increasing cost of drilling for dynamite blasting to generate acoustic noise and the present invention primarily relates to this method of generating acoustic energy, although it is not so limited.
However generated, the acoustic energy travels downwardly through the earth and is reflected at the interfaces between differing layers of rock and returns upwardly to the earth's surface. The return of the waves of acoustic energy is detected by one or more geophones placed on the earth's surface. The time taken for the wave to travel downwardly, be reflected, and return upwardly, is measured and the time is proportional to the depth of the interface from which the wave was reflected between successive layers of rock beneath the earth's surface. Additional time measurements are then made with the source of acoustic energy and the detector spaced a distance from the first; comparison of the times taken by the first and second waves yields an indication of the change in the depth of the interface between the rock layers as the source and detectors move from one location to the next. If the outputs of the detectors are properly processed and are graphed in accordance with the relative positions of source and detector, a representation of a cross-section of the earth results, on which the interfaces between rock layers appear as deflections on the individual output lies, or "traces".
A great deal of prior art has been concerned with the problem of eliminating noise from the electrical records output by the geophones. One of the most common techniques is to provide a plurality of measurements of the time taken for acoustic waves to travel downwardly from a plurality of sources of acoustic energy located at differing locations, reflect from substantially the same point on the interface between rock layers, and reflect upwardly to a like plurality of geophones. Since in general the angle of incidence of the acoustic wave on the interface and its angle of reflection are equal, if the spacings of the geophones and of the sources are kept centered about a point on the earth's surface, it may be assumed that the reflections are from substantially the same point on the interface. If the resultant traces are then correlated, that is, if an event common to each of the records of the plurality of geophone outputs are aligned, thus correcting for the longer travel distance of a wave from an acoustic energy source located a longer distance from the point of reflection, and are then summed, the noise in the records, being random, will tend to cancel out, while the record of the reflection will be made stronger, thus increasing the signal-to-noise ratio of the record. This "common depth point" (CDP) technique is very widely used and is thoroughly described in the prior art. See, for example, U.S. Pat. No. 3,381,266 to Harris.
Most seismic exploration using the CDP technique has been done so as to provide a record indicative of a cross-section of the earth taken along a line of exploration. That is, the exploration has been generally linear. See U.S. Pat. No. 3,240,286 to Musgrave. However, it has been increasingly desirable to provide a more detailed picture of the sub-surface of the earth; hence three-dimensional techniques have more recently been developed. See for example, U.S. Pat. No. 3,867,713 to Tegland et al. In the Tegland et al technique, a plurality of arrays of detectors are disposed along lines forming an angle, preferably a 45.degree. angle, to an overall line of exploration. To the extent that the lines of detectors broaden the line of exploration, this technique can yield a three-dimensional picture of the sub-surface data. However, the Tegland et al technique is limited in that it remains a linear technique, only broadening the line surveyed and hence frequently does not yield sufficiently detailed data to obtain a full picture of the sub-surface of the earth being surveyed. In addition to this deficiency of the Tegland et al patent, it will be appreciated by those skilled in the art that in general it is desirable to have as many records of reflections from a common depth point as possible so as to further increase the signal-to-noise ratio. The Tegland technique has only a limited number of records per common depth point surveyed. Other three-dimensional seismic data recordation schemes, such as in U.S. Pat. Nos. 3,529,282 to Brown et al and 3,597,727 to Judson et al suffer from the same deficiencies.